The trial over the Trump administration’s plan to ask about citizenship on the 2020 U.S. Census began on Monday with expert testimony explaining how the change may cause an undercount on the national survey among Hispanic households, as well as those of noncitizens.

Attorneys for the U.S. Department of Justice, meanwhile, began their effort to rebut the New York attorney general’s challenge by trying to poke holes in the research and prior testimony about the effect of a question on immigration status.